Animals that are unable to regulate their body temperature instead depend directly on the environment are called poikilotherms or cold-blooded animals,
They have a wide range of temperature fluctuations.
They use their external environment such as basking in the sun, swimming in the water, etc to control their temperature.
All amphibia, reptiles, some fishes, and many invertebrates are poikilotherms.
Turtles are poikilotherms as they belong to the class reptilia.
They have no internal mechanism to regulate their temperature.